Neutral Hydrogen in the Vicinity of Open Clusters

Hi observations of twenty-eight open clusters, having optical diameters «= 20' and -034 «= (B - V)T «= +0.40, have been made with the National Radio Astronomy Observatory 300-foot telescope (beam width 12') and the 100-channel autocorrelation receiver (velocity resolution 2 km sec1). No attempt was made to associate large-scale features (»= 10 in angular size) with a cluster; rather, we looked for detailed correlations between the cluster and features of the H i gas (either maxima or minima) in the immediate vicinity of the cluster. The anticipated location of the associated H i feature in velocity space was taken to be the optically determined stellar radial velocity, when available, or otherwise the velocity calculated using a galactic-rotation model On the average, a cloud of »= 30 Mo should be detectable (»= 1 to »= 200 M~ in the best and worst cases, respectively). At the position `and velocity of some clusters (especially NGC 581, 663, 744, and 1893), we do find hydrogen maxima or minima, but there are no more of these cases than would be expected on the basis of chance. For the average cluster the amount of associated neutral hydrogen, if it is present at all, is certainly less than one-third its total stellar mass